Numerous solid-state materials have heretofore been employed as detectors for ionizing radiation. Such semiconductor detectors have included cryogenically cooled silicon and germanium crystals which are particularly useful as energy-dispersive x-ray detectors in spectrochemical analysis.
The commonly accepted energy-dispersive x-ray detector is presently lithium-drifted silicon, and spectrometers employing this detector have the ability to simultaneously detect x-rays from most of the elements in the periodic table. However, the need to keep such detectors cooled with liquid nitrogen has posed substantial impediments to the use of energy-dispersive x-ray fluorescence systems in many applications. Thus, it has been a desideratum to develop a detector for extreme short wavelength radiation which combines the advantages of room-temperature operation, such as found in scintillation and proportional counters, with the excellent spectral energy resolution of cryogenically cooled spectrometers.
Various materials have been considered for the detection of radiation at room temperature. In the field of x-ray detection, good results have been obtained with semiconductors formed from gallium arsenide and cadmium telluride. However, these crystals present problems with regard to their transport properties and band gap sizes and thus do not provide detection with a sufficiently low dark current which is detector noise. Other solid-state compound crystals lack either good enough charge carrier transport or a large enough band gap or both. Mercuric iodide crystals have been shown to combine the advantages of good electron transport properties with a large enough energy band gap so that detectors with very low leakage current at room temperatures may be formed. Unfortunately, the yield and reproducibility in preparing satisfactory crystals is low and under inadequate control.
Relatively pure mercuric iodide undergoes a reversible phase transformation at 130.degree. C. from a tetragonal structure to an orthorhombic structure. Crystals in the room temperature tetragonal phase are red, whereas the high temperature phase is characterized by yellow crystals. The tetragonal crystals provide the desired semiconducting effect. Since the melting point of mercuric iodide is about 250.degree. C., the phase transformation prevents the growing of the desired tetragonal crystals by solidification of its liquid. Although it is possible to grow crystals from solution, growth from a vapor phase yields crystals to date with vastly superior electrical transport properties. Various vapor-growth techniques are known in the art. J. Saura et al., J. Cryst. Growth, 15 (1972) 307; H. Scholtz, Acta Electronics, 17 (1974) 69; and S. P. Faile et al., J. Cryst. Growth, 50 (1980) 752.
While these methods may yield platelets of a size and shape suitable for detector fabrication, the quality of such crystals has not been reliable, i.e., usable crystals may constitute as little as one percent of the crystals formed.
While I do not wish to be bound by any particular theory, it appears that the presence of various impurities in reagent-grade mercuric iodide is responsible for the problems heretofore described. The metallic impurities commonly present in tetragonal mercuric iodide are in the concentration range of a few to approximately 1,000 parts per million (ppm). These impurities include silicon, aluminum, the transition elements and elements from groups I and II. In addition, the presence of mercuric oxide or mercuric hydroxide impurities in trace amounts is a likely source of the wide range of structural phase transition temperatures near 130.degree. C. In addition, the sublimation and crystal growth of mercuric iodide in a closed system in which the fugacity of oxygen may be as high as 10.sup.-6 to 10.sup.-3 atmospheres leads to the formation of mercuric oxide until the oxygen is essentially consumed, to yield a mercuric iodide: mercuric oxide dilute solid solution.
Reagent grade mercuric iodide contains an assortment of trace quantities of metallic impurities, trace amounts of organic solvents and one or more of the following oxygen containing compounds: HgO, Hg(OH).sub.2, Hg.sub.2 O, HgI(OH), Hg.sub.2 (OH).sub.2 and Hg.sub.2 (OH)I. While these impurites may be reduced by repeated sublimation of the starting material, the effectiveness of this method is highly variable. Gross deviations (in terms of the ppm concentrations of defects) cause a one degree variation in the temperature at which the structural phase transition occurs, and thus effects crystal density and other properties.
According to the present invention, methods are provided for the purification of mercuric iodide and the growth of pure tetragonal mercuric iodide single crystals.
Mercuric iodide is purified in an open-tube system by vapor transport in an inert gas with deliberate additions of oxygen or iodine, alone or in combination. Impurities are removed by vaporizing mercuric iodide in a flowing stream of essentially inert gas in the presence of an oxygen species in an amount required to form an oxidizing vapor concentration greater than the combined vapor concentrations of the metallic and organic impurities. It is preferred to include a vapor pressure of iodine in the inert gas stream in order to minimize the formation of undesired mercuric oxides and favor the conversion of any oxygen-containing mercury compounds in the sample to mercuric iodide, the iodine vapor pressure being greater than that of the oxidizing vapor concentration. Thus, impurities such as metallic iodides are reacted to form oxides which are less volatile than mercuric iodide, and impurities such as organic residues are oxidized to form more volatile products, both of which are separated from the purified mercuric iodide vapor by gradient crystallization.
In another aspect of the invention, mercuric iodide containing small amounts of mercuric and mercurous oxides and hydroxides is purified by passing the vaporized mercuric iodide through a reaction zone in an essentially inert gas stream in which the thermodynamic equilibria and kinetics favor the conversion of oxides to iodides. A reducing species is added in an amount necessary to provide a reducing vapor pressure in excess of any oxidizing vapor pressure, thus converting oxygen containing mercury compounds to mercuric iodide.
In yet another aspect of the invention, tetragonal mercuric iodide crystals which are stable at room temperature are grown by a vaporizing mercuric iodide in an atmosphere which is essentially free of oxygen species by the addition of an excess reducing vapor pressure such as by the inclusion of one or more of the group consisting of iodine, hydrogen, hydrogen iodide or carbon monoxide, in order to limit the oxidative effect of residual oxygen containing substances which may be found in the growth environment.